comments capture information from designer mind
The overall idea behind comments is to capture information that was in the mind of the designer but couldn’t be represented in the code. This information ranges from low-level details, such as a hardware quirk that motivates a particularly tricky piece of code, up to high-level concepts such as the rationale for a class. When other developers come along later to make modifications, the comments will allow them to work more quickly and accurately. Without documentation, future developers will have to rederive or guess at the developer’s original knowledge; this will take additional time, and there is a risk of bugs if the new developer misunderstands the original designer’s intentions. Comments are valuable even when the original designer is the one making the changes: if it has been more than a few weeks since you last worked in a piece of code, you will have forgotten many of the details of the original design